There was an unexpected four-legged star on Broadway on Saturday when Jessica Chastain’s dog escaped from backstage at “The Heiress.” “Her three-legged rescue- dog, Chaplin, walked onto the stage during a dramatic scene,” said a spy. Luckily the “Zero Dark Thirty” star is as good a trainer as an actress. “Without missing a beat, Jessica walked offstage, knowing her dog would follow, and she just continued the scene.” At the end of the matinee, Chastain took a final bow holding the fluffy white pooch.